This web page summarizes the results of the search for the lepton flavor violating decays Bsd->eμ. These decays are forbidden within the original Standard Model of particle physics in which leptons do not change flavor. For example, a muon would never turn into an electron. If this were observed, it would be a signal for new physics. The observation of neutrino oscillations has shown that lepton flavor changes actually occur in nature and therefore the standard model needs to be modified to incorporate this. This analysis represents a search for lepton flavor changes in a different channel.
CDF
performed this measurement in Run-I
[PRL(81)
1998].
The Run I measurement was an absolute measurement using
an
electron-muon triggered sample with an integrated luminosity of 102 pb-1
. For Run II no suitable electron muon trigger exists,
therefore
we employ a different measurement strategy. In Run-II, we perform
a relative measurement using samples collected by
the
two-track SVT trigger and using
candidate
Bd->Kπ events collected with
the same trigger path for a relative normalization. The
branching ratio Br(
Bd->Kπ)
is measured to be 19.4 +/- 0.6 x 10-6.
This
way, many of the common reconstruction efficiencies due to tracking,
triggering, and vertexing, cancel. The remaining effects that still
have to be considered are the relative detector and kinematic
acceptance including the mass window requirement and the electron and
muon identification efficiencies. To see all the
ingredients
that need to be measured to perform this measurement and for
the
formula relating the number of observed candidate events to the
branching ratio Br(Bs→eμ) click here
Once
we obtain the limit on the branching ratios we can use this result to
constrain the masses of leptoquarks predicted by a specific
theoretical model. The
decays Bsd->eμ
are possible
in some theoretic models containing
lepton-flavor violation tree-level couplings mediated by
leptoquarks. The Grand Unification Theory (GUT) by J. Pati and A. Salam
predicts spin 1 gauge bosons the so called “Pati Salam
Leptoquarks” ( PSLQ) that carry both color and
lepton quantum
numbers
[PRD 10,275
(1974)] (see Feynman diagram below). It is the simplest model based
on the symmetry group SU(4)c where the
lepton number is the fourth “color” . Within the Pati-Salam model, one
can derive a relationship between Br(Bs→eμ)
and
the leptoquark mass (see G. Valencia, S.
Willenbrock
[PRD 50,(1994)]). We will use this relationship to constrain
the
leptoquark masses. For the function relating the branching ratios to the Pati Salam leptoquark masses click Bs or Bd
.
Using the same technique as for the Bsd->eμ search (with the difference that both tracks are required to be identified as an electron) we also performed a search for the decay Bsd->e+e-. This decay can occur within the standard model at a very low rate.
With 2fb-1 of CDF run II data we measure :
(all results are
Bayesian limits given at 90 (95)% confidence level)
| Br(Bs→eμ) | < | 2.0(2.6) x 10-7 |
| MLQ(Bs) | > | 47.7(44.6) TeV |
| Br(Bd→eμ) | < | 6.4(7.9) x 10-8 |
| MLQ(Bd) | > | 58.6(55.7) TeV |
| Br(Bs→e+e-) | < | 2.8(3.7)x 10-7 |
| Br(Bd→e+e-) | < | 8.3(10.6)x 10-8 |
